FTIR vs Dispersive IR
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This document compares and contrasts the properties of dispersive infrared (IR) spectroscopy and Fourier transform infrared (FTIR) spectroscopy. It discusses seven key differences between the two techniques: 1) components and movement, 2) calibration, 3) stray light effects, 4) number of frequencies detected simultaneously, 5) scan speed, 6) effects of IR radiation from the sample, and 7) advantages of single beam versus double beam optics.
FTIR vs Dispersive IR
- 1. PROPERTIES DISPERSIVE IR FTIR 1.Components Many moving parts result in mechanical slippage. Only mirror moves during an experiment 2.Calibration Calibration against reference spectra required to measure frequency Use of laser provides high frequency precision (to 0.01 cm-1 (Connes’ advantage) 1.Stray light Stray light within instrument causes spurious readings Stray light does not affect detector, since all signals are modulated. 2.Resolution In order to improve resolution, only small amount of IR beam is allowed to pass through the slits. Much larger beam aperture used; higher energy throughput (Throughput or Jacquinot’s advantage). 3.No. of Frequency Only narrow-frequency radiation falls on the detector at any one time. All frequencies of radiation fall on detector simultaneously; improved S/N obtained quickly (Multiplex or Fellgett’s advantage) 4.Scan speed Slow scan speeds make dispersive instruments too slow for monitoring systems undergoing rapid change (e.g. GC effluents) Rapid scan speeds permit monitoring samples undergoing rapid change. 5.Source effects Sample subject to thermal effects from the source due to length of scan time. Short scan times, hence sample is not subject to thermal effects. 6. IR from sample itself Any emission of IR radiation by sample will fall on detector due to the conventional positioning of the sample before the monochromator. Any emission of IR radiation by the sample will not be detected. 7.Advantage of Beam optics Double beam optics permit continuous real-time background substraction. Single beam optics; background spectrum collected separately in time from sample spectrum. Can result in error if background spectra not collected frequently.